1. Field of the Invention
The present invention relates to a nonvolatile semiconductor storage device and more particularly to a nonvolatile semiconductor storage device in which the gate electrode structure of transistors that form memory cells is improved and a method of manufacture thereof.
2. Description of the Related Art
Nonvolatile semiconductor memories which use metal oxide nitride oxide semiconductor (MONOS) cells for memory cell transistors are structured such that a tunnel insulating film, a charge storage layer, a block insulating film and a gate electrode are stacked on the device areas of a semiconductor substrate. Data of the MONOS cell is erased by injecting holes from the semiconductor substrate through the tunnel insulating film into the charge storage layer. At this point, electrons will be injected from the gate electrode through the block insulating film into the charge storage layer, causing a problem that sufficient erasing cannot be accomplished.
To solve the problem, conventionally, use has been made of a high-k film which has a larger dielectric constant than the tunnel insulating film as the block insulating film and a metal which has a larger work function than silicon as the gate electrode.
To prevent the short-channel effect of the memory cell transistors, it is required to increase the dimension in the direction of gate length of the gate electrodes. In addition, where a process in which the gate electrodes are oxidized is included, it is required to further increase the dimension in the direction of gate length of the gate electrodes. However, increasing the dimension in the direction of gate length of the gate electrodes becomes reducing space between adjacent gate electrodes. As a result short-circuiting of adjacent gates can occur.
A proposal has been made for a device which is not the MONOS structure but one of the nonvolatile semiconductor memories having a stacked gate structure consisting of a floating gate and a control gate and which is formed such that the control gate is formed from two or more layers and the top electrode layer is formed with a film of oxide on the side (for example, Jpn. Pat. Appln. KOKAI Publication No. 2008-53295). With this device, however, an oxide film (sidewall insulating film) is also formed on the floating gate side, offering a problem that the gate length is reduced correspondingly. Furthermore, it is required to form an anti-oxidation film between the two gate layers which serve as the control gate, which makes the structure complex. Therefore, application of the structure disclosed in the publication to the MONOS structure fails to solve the aforementioned problems.